1,1-disubstituted ethylene monomers and compositions containing the same are well known and, for the most part, widely available. They have utility in a broad array of end-use applications, most notably those which take advantage of their cure or polymerizable properties. Specifically, they find broad utility in coatings, sealants and adhesives, among other applications. Those 1,1-disubstituted ethylenes having one or, preferably, two electron withdrawing substituents at the 1 position have been used to provide adhesives and sealants with rapid cure rates and high bond strengths. Most notable among these are the cyanoacrylates such as ethyl cyanoacrylate, butyl cyanoacrylate and octyl cyanoacrylate. Another class of 1,1-disubstituted ethylenes that have demonstrated a lot of promise, but have limited, if any, commercial success are the methylidene malonates, including diethyl methylidene malonate.
Commercial success of the 1,1-disubstituted ethylenes is reliant upon a number of variables and factors including reasonable cost, high purity, good (especially long) shelf life and rapid cure rate. In an effort to achieve these goals, much work has been done to develop new and/or improved processes and synthetic schemes for their manufacture, purification and isolation.
For example, α-cyano acrylates have been prepared (U.S. Pat. No. 6,245,933) by reacting a cyanoacetate such as ethyl cyanoacetate with formaldehyde or a formaldehyde synthon such as paraformaldehyde in a Knoevenagel condensation followed by transesterification. The product mixture is then cracked and distilled to produce the α-cyano acrylate monomer.
In another preparation (U.S. Pat. No. 7,569,719), ethyl cyanoacetate is reacted with specific iminium salts prepared from paraformaldehyde and primary amines such as t-butyl amine.
In a process disclosed in WO 2010129066, crude diethyl methylidene malonate is prepared from diethyl malonate and paraformaldehyde, subsequently stabilized and distilled, then restabilized and redistilled.
The large amount of work done to explore various synthetic routes and the extremes taken to purify the 1,1-disubstituted ethylenes demonstrate both the importance and the complexity of these materials and their production. While commercial success has been found with the cyanoacrylates, methylidene malonates continue to struggle. Indeed, methylidene malonates have been the subject of decades and decades of efforts to produce them in commercial quantity at a commercially viable price and of suitable cure speed and shelf stability; yet, frustration continues.
Two of the earliest methods for the production of dialkyl methylidene malonates, the simplest of the methylidene malonates, were the iodide method in which methylene iodide was reacted with dialkyl malonates and the formaldehyde method in which formaldehyde was reacted with dialkyl malonates in the presence of a base, in solution in alcohol solvents. The former was unsatisfactory due to very low yield and expensive starting materials. The latter, though periodically giving better yields than the iodide process, gave relatively poor yields and, more critically, was widely inconsistent from batch to batch, even under the same conditions.
Despite its problems, early efforts continued to focus on the formaldehyde method. One of the most widely practiced formaldehyde methods consisted of reacting diethyl malonate with formaldehyde in glacial acetic acid in the presence of a metal acetate catalyst to produce the diethyl methylidene malonate. The latter was subsequently recovered by distillation following removal of the catalyst by filtration and separating off the solvent. These efforts continued to frustrate and various modifications and iterations of this basic process were developed all in an effort to improve the consistency and yields associated therewith.
As an alternative, Bachman et al. (U.S. Pat. No. 2,313,501) taught the reaction of a C1-C5-dialkyl malonate with formaldehyde in the presence of an alkali metal salt of a carboxylic acid, in solution in a substantially anhydrous carboxylic acid solvent, followed by fractional distillation to separate the desired product. Bachman et al. indicate that their process is advantageously carried out in the presence of inhibitors of the polymerization of monomeric vinyl compounds. Suitable inhibitors are said to include the copper salts such as copper chloride and, especially, copper salts of carboxylic acids such as cupric acetate, iron salts such as ferric acetate, and phenols, such as hydroquinone. These are added to the solution mix before the addition of the malonate.
Although Bachman et al. reported yields of up to 72%, the results presented are conversion rates, not yields. Looking at the actual yields of the process, Bachman et al.'s best performance was a yield of 43% with all others being less than 25%. Though Bachman et al. speak of high purity and the ability to recover pure material, they never present any details or data as to what those purities or recoveries were. In any event, Bachman et al. reported that the isolated product, upon standing, polymerized in a day to several weeks time depending upon the purity of the isolated material, which polymer was then heated to a sufficiently high temperature to facilitate the reversion of the polymer to the monomer.
D'Alelio (U.S. Pat. No. 2,330,033), on the other hand, alleged that such processes were erratic and more often produced yields that averaged 10 to 12 percent. D'Alelio espoused an improved process with yields on the order of 30% and higher by reacting a malonic acid ester with formaldehyde in a ratio of one mole of the former to at least one mole of the latter under alkaline conditions and, in most cases, in the presence of a polymerization inhibitor such as copper, copper acetate, hydroquinone, resorcinol, or catechol, to form a methylol derivative. The methylol derivative was then acidified to a pH below 7.0 using a suitable organic or inorganic acid in order to retard further reaction. The acidified mass is then dehydrated to form the corresponding methylidene malonate which is subsequently separated by distillation.
Coover et al. (U.S. Pat. No. 3,221,745 and U.S. Pat. No. 3,523,097) took yet another approach to the formation of the methylidene malonates, electing to begin with a preformed dialkyl alkoxymethylenemalonate. In accordance with their process, the olefinic double bond of the latter compound was subjected to hydrogenation in the presence of a hydrogenation catalyst and the hydrogenated compound was then subject to pyrolysis in the presence of a phosphorous pentoxide inhibitor to strip off the alcohol to produce the methylene malonate. The resultant mass was then subjected to vacuum distillation at low temperature to separate an allegedly high purity methylidene malonate, though with a low yield. According to Coover et al., the use of low temperature distillation is said to prevent the contamination of the monomer with pyrolytic products that commonly result from high temperature distillation. These high purity monomers are said to be especially important for surgical applications.
Eventually, such efforts led to multi-step processes in which certain unsaturated molecules served as a platform for the formation of intermediate adducts from which the methylidene malonates were subsequently stripped and recovered. For example, Hawkins et al. (U.S. Pat. No. 4,049,698) found that certain malonic diesters could be reacted with formaldehyde and a linear, conjugated diene in the presence of a primary, secondary or tertiary amine at about reflux temperature to form an intermediate adduct that could then be readily pyrolyzed at temperatures in excess of 600° C. to split off the desired methylidene malonate. Similarly, Ponticello (U.S. Pat. No. 4,056,543) and Ponticello et al. (U.S. Pat. No. 4,160,864) developed processes by which asymmetrical methylene malonates, especially methyl allyl methylene malonate, were prepared from previously formed norbornene adducts, the latter having been prepared by the Diels-Alder reaction of an alkyl acrylate with cyclopentadiene at room temperature or with heating or use of a Lewis catalyst. The so formed monoester norbornene adducts were then reacted with an electrophile material in the presence of an alkyl-substituted lithium amide complex to form the diester adduct and subsequently pyrolyzed at a temperature of 400° C. to 800° C. at a pressure of 1 mm to 760 mm Hg in an inert atmosphere to strip off the desired methylene malonates. These efforts, despite their gains in yield and/or purity, still failed to achieve commercial success.
Citing numerous disadvantages of the foregoing processes, which disadvantages were said to make them difficult, if not impossible, to adapt to industrial scale, Bru-Magniez et al. (U.S. Pat. No. 4,932,584 and U.S. Pat. No. 5,142,098) developed a process whereby anthracene adducts were prepared by reacting mono- or di-malonic acid ester with formaldehyde in the presence of anthracene, most preferably in a non-aqueous solvent medium in the presence of select catalysts.
According to Bru-Magniez et al., the anthracene adducts were said to be readily produced in high yields with the desired methylidene malonates obtained by stripping them from the anthracene adduct by any of the known methods including heat treatment, thermolysis, pyrolysis or hydrolysis; preferably heat treatment in the presence of maleic anhydride. The resultant crude products were then subjected to multiple distillations, preferably lower temperature distillations under vacuum, to recover the purified methylidene malonate. Despite the claim to high yields, their crude yields were generally in the range of 21-71%, more importantly, nothing is said with respect to the purity of the material obtained.
Based on conversations with the successors to the Bru-Magniez technology, efforts to commercially produce the material have met with great difficulty owing to the high instability of the overall production process and final products. Indeed, they reported a high failure rate: of the limited batches that actually survived through crude distillation, the resultant products had to be stored in a freezer even after stabilizing with upwards of 50,000 ppm SO2 due to their high instability and spontaneous polymerization.
Malofsky et al. (WO 2010/129068) solved some of the problems associated with process instability of the Retro-Diels-Alder adduct process by using polymerization inhibitors concurrent with or prior to stripping of the adduct. Inhibitors such as trifluoroacetic acid and hydroquinone were used. In some examples, trifluoroacetic acid was also added to the distillate. Only limited curing studies were done, but the resultant malonates were able to be polymerized with tetrabutylammonium fluoride. Malofsky teaches a variety of purification processes including double distillation and extracting the product with an alkane such as n-heptane. Although this is an improvement over the art, these various purification processes can be costly and can reduce yield.
The problems of producing 1,1-disubstituted ethylene with a good combination of cost, purity, shelf life and rapid cure rate is not specific to methylidene malonates or to a particular method of production. Imohel et al. (U.S. Pat. No. 3,728,373) teaches the use of hydroquinone and other polymerization inhibitors with polycyanoacrylates and decomposes these and distills cyanoacrylates from the polymer/stabilizer mixture. More recently, Malofsky et al. (U.S. Pat. No. 6,512,023) theorized that the stability of 1,1-disubstituted ethylene monomer and polymers could be improved by the use of specific combinations of certain vapor phase and certain liquid phase anionic polymerization inhibitors. These monomers, typically cyanoacrylates, exhibited improved stability. McArdle (U.S. Pat. No. 7,973,119) prepares electron deficient 1,1-disubstituted ethylene compounds such as cyanoacrylates and methylidene malonates by an entirely different process using protonated imines. McArdle teaches the use of free radical and acidic stabilizers.
The large amount of work as well as the numbers of scientists who have endeavored to deal with these problems is a testament to the perceived importance of 1,1-disubstituted ethylene monomers, including methylidene malonates and cyanoacrylates, and the complexity involved in the production thereof, most especially the methylidene malonates, and the use thereof. Reaction viability (i.e., the ability to isolate product before cure), low yields, low purity, poor shelf life or the need for extreme storage conditions, and cure rate are erratic and have and continue to frustrate those efforts to overcome the same. For sure, improvements in stabilization techniques and technology, both of the final product and in the isolation thereof, have dramatically improved reaction viability, yields, purity and shelf life; however, cure speed remains erratic and unpredictable. While, from a purely technical perspective cure speed may not be critical; from a commercial perspective it is highly critical. Commercial utility is severely limited, if not wanting, and the prior improvements are largely meaningless, if one is unable to show or establish consistent and/or predictable cure speeds. Curable compositions need defined cure speed or cure speed parameters for utility in commercial, medical, and industrial applications.
However, a discontinuity exists in that the two desired properties, namely excellent shelf life and a rapid or predictable cure rate are often at cross purposes and one must be compromised at the expense of the other. Excellent shelf life implies good stability and a resistance to polymerization whereas a rapid cure rate implies a highly active and, hence, unstable, product whose tendency is to rapidly polymerize. Additional complexity in the production of these monomers lies in the fact that many purification techniques involve high temperature, e.g., distillations, and/or conditions which accelerate polymerization and/or increase instability of the material being worked upon. As a result, polymerization and/or degradation of the materials being worked on can occur. Indeed, the production of cyanoacrylate takes advantage of this effect, allowing the monomer to polymerize and then cracking the monomer in the presence of certain stabilizers, to effectively regenerate the monomer. However, this process is not amenable to or suitable for the production of most 1,1-disubstituted ethylenic monomers, especially the methylidene malonates.
Thus, there remains a need for a new and/or improved process for the production of 1,1-disubstituted ethylenic monomers, especially methylidene malonates, which provides consistent production viability with high yields and purity and good shelf life as well as rapid and consistent or predictable cure speeds.